1. Field of the Invention
This invention relates to receivers for receiving signals which comprise a mixture of signals from a plurality of sources where two of the signal sources use different coding schemes from each other. In particular this invention relates to receivers for receiving spread spectrum signals which also include unwanted narrowband signals, for example, radio signals which comprise a mixture of signals from signal sources using the Group Special Mobile (GSM) ETSI standard coding and from signal sources using Code Division Multiple Access (CDMA) coding.
2. Description of Related Art
Demand for systems and services which require the use of the radio spectrum has recently far outstripped the capacity of present systems. This means that future systems must provide significantly improved spectral efficiency. One technique which has been suggested is to allow wideband spread spectrum signals to share a common spectrum with conventional narrowband signals.
Spread spectrum signals use a bandwidth which is in excess of the minimum bandwidth required to transmit the information in the signal. One method of generating a spread spectrum signal is known as pseudo random noise modulation (or direct sequence modulation). The spectrum of a signal is spread using a spreading code at the transmitter. A corresponding receiver uses the same code to despread the spectrum. The technique used to despread the spectrum to regenerate the original data signal has the effect of spreading the spectrum of any other signals which are not correlated with the spreading code used by the transmitter. Where, as is usual, several wideband signals from different sources (using different codes) share the same frequency spectrum, this technique is known as Code Division Multiple Access (CDMA).
The transmitted narrowband signal generally has a high power spectral density compared with that of the transmitted wideband signal. In the narrowband signal receiver the wideband signal creates interference in the narrowband signal. Equally, once the combined received signal has been decoded by a spread spectrum receiver, the narrowband signal causes interference in the decoded spread spectrum signal.
One method which has been used to reduce the interference between the narrowband signal and the wideband signal is to use corresponding filters in the spread spectrum transmitter and receiver to attenuate those frequencies used by the narrowband signal. A notch filter in the spread spectrum transmitter reduces the interference in the narrowband signal from the wideband signal. A corresponding notch filter in the spread spectrum receiver attenuates the frequencies used by the narrowband signal prior to despreading the spectrum to decode the spread spectrum signal.
There are problems with using such techniques to reduce the cross signal interference. To minimise the signal removed by the attenuation filters which is outside the band used by the narrowband signal, filters with very sharp cut-offs are required.
However, in a system such as the GSM cellular network adjacent cells have transmitters using different frequencies. Therefore the receiver is required to receive different frequency bands according to which transmitter is being used. Filters which are required to attenuate these narrowband frequencies need to be programmable. Programmable filters are most easily achieved with digital hardware which implies the use of high order finite impulse response (FIR) filters. Such filters currently only work on signals up to a maximum of a few tens of megahertz so it is most convenient to implement such filtering at baseband.
Milstein and Schilling. xe2x80x9cThe CDMA Overlay Conceptxe2x80x9d, International Symposium on Spread Spectrum Technologies and Applications 1996, disclose a spread spectrum transmitter and receiver which utilise a notch filter. The receiver performs filtering simultaneously with despreading at baseband.
Any non-linear distortion to the signal prior to attenuating those frequencies used by the narrowband signal means that any signal in the narrowband frequency range introduces noise outside that range. The power spectral density of the narrowband signal is typically many tens of dB higher than the power spectral density of the wideband signal. Any components between the radio frequency receiver and the filter must have a very large dynamic range in order to minimise noise introduced into the remaining wideband signal by such non-linear distortions.
If a non-linear coding method is used to increase the dynamic range of such components then there are resolution problems for the low-level wideband signal.
Therefore the problem with known systems which utilise an attenuating filter, is that performing filtering at baseband requires components prior to the filtering step to have a very large dynamic range, but filtering at high frequency is not possible using currently available digital programmable filters.
According to the present invention there is provided an apparatus for decoding a spread spectrum signal comprising
means for generating a despreading signal for despreading the spectrum of a signal;
means for despreading the spectrum of a signal using the despreading signal;
means for downconverting a signal to produce a lower frequency signal; and
means for attenuating predetermined frequencies of a signal;
wherein the despreading means is connected to despread the spread spectrum signal prior to downconverting and the attenuating means is connected to attenuate predetermined frequencies of the spread spectrum signal prior to downconverting.
Preferably the received signal is a composite signal comprising a spread spectrum signal component and a narrowband signal component which has a much higher power spectral density than the spread spectrum signal component and the attenuated frequencies are those present in the narrowband signal component. Optionally the spectrum is de-spread using a pseudo noise code.
Advantageously, the attenuating means is connected to attenuate predetermined frequencies of the despreading signal and the resulting notched despreading signal is connected to simultaneously despread the spread spectrum signal and attenuate the predetermined frequencies of the spread spectrum signal.
According to another aspect of the invention there is provided an apparatus for decoding a spread spectrum signal comprising means for generating a despreading signal for despreading the spectrum of the signal; and attenuating means connected to attenuate predetermined frequencies of the despreading signal to provide a notched despreading signal; characterised in that the apparatus further comprises mixing means connected to mix the notched despreading signal with a signal from a local oscillator to provide an upconverted notched despreading signal; and mixing means connected to mix the upconverted notched despreading signal with the spread spectrum signal.
According to another aspect of the present invention, there is provided an apparatus for decoding a spread spectrum signal which has been transmitted via a transmission channel, said signal comprising a wanted spread spectrum signal component and an unwanted narrowband signal component, the narrowband signal having been coded using a coding scheme with error correction capabilities, said apparatus comprising a narrowband signal subtractor containing
means for decoding and correcting the composite signal according to the narrowband signal coding scheme to provide a decoded version of the narrowband signal;
means to estimate the amplitude and phase characteristics of the transmission channel;
means to encode the decoded corrected signal according to the narrowband signal coding scheme to produce an estimated narrowband signal;
means to adjust the phase and amplitude of the estimated narrowband signal according to the estimated amplitude and phase characteristics; and
means to subtract the adjusted signal from the received composite signal to provide an estimate of the spread spectrum signal component.
Preferably the narrowband coding scheme can also detect uncorrectable errors and in which the decoding means is arranged upon detection of an uncorrectable error to suppress the said subtraction.
In another embodiment there is also included means for attenuating the frequencies of the signal corresponding to the narrowband signal component wherein the decoding means is arranged upon detection of an uncorrectable error to cause the attenuating means to be used.
Corresponding methods for decoding spread spectrum signals are also provided.
Embodiments of the present invention will be described, by way of example only, with reference to the drawings where